Multiple beam lasing systems can be used in a variety of applications, one of which is in a printing system where rotating polygonal mirrors are used to scan the light beams across a photoreceptive surface. The use of multiple laser beams in a printing system provides the capability of producing more than one line of information at a time, thus enabling high pel resolution, for example, 480 pels per inch or higher while keeping practical speeds for the rotating polygonal mirror. Multiple beam devices provide other capabilities as well; that is, the multiple beams can be used to alter the shape of the effective writing spot by modulating the spots within the spot group or they can be used to modulate the amount of light provided at each pel position.
While the use of multiple beam lasing systems have significant advantages over a single beam lasing system, multiple beam systems employing discrete laser sources or employing laser array chips require precise mechanical alignment to assure that a print position (pel location) is properly located from line to line, that is, that a pel written by one laser is properly aligned with a pel written by another laser. Multiple beam systems which employ laser arrays require that the lasing diodes be placed on the chip to close tolerance, nevertheless, there can be some physical misplacement of the semiconductor laser diodes within the chip making it desirable to provide a system which can utilize laser arrays with some laser spot variation. Additionally, laser array chips are typically mounted in a tilted fashion to provide correct beam alignment in the non-scan direction. However, the tilt creates an offset from beam to beam in the scan direction creating pel placement problems. As used herein, the term mechanical misalignment includes: (1) physical variation in the placement of lasing spots on a laser array chip; (2) mechanical misalignment of the laser array chip or of discrete lasing sources within a machine; and (3) the offset produced by tilting a laser array.
Still another problem with multiple beam lasing systems is wavelength variation from laser beam to laser beam. In laser arrays, wavelength variation is often within a few nanometers, but can be significant enough to create pel placement problems. In addition, it is desirable to utilize laser arrays with a wider range of wavelength variation in order to produce low cost systems. Such considerations are also true of multiple beam systems utilizing discrete laser sources. In any event, even if lasing sources are carefully matched, temperature and age can create dynamic wavelength variation which can destroy the accuracy of the system. It should be noted, that laser beam position is typically amplified through printhead optics, such that a small misalignment at the laser chip or a small wavelength variation can result in a much larger error at the focal plane.
Electronic correction schemes in the prior art have been designed to achieve multiple pel corrections for misalignments due to a necessary tilting of a laser array. Additionally, some of these electronic schemes provide sub-pel accuracy Such systems require nanosecond capability when high resolution systems are contemplated, that is, 480 pels per inch or higher. Clock frequencies of 500.times.10.sup.6 Hz are required to resolve to two nanoseconds. Such a clock and the high speed logic utilizing such resolution are expensive.
It is an object of this invention to provide an electronic correction scheme for use with a multiple beam scanning laser system to provide correction for mechanical misalignment errors.
It is also an object of this invention to provide an electronic correction scheme for use with a multiple beam scanning laser system to provide correction for chromatic errors.
It is another object of this invention to quantify misalignment errors separately from chromatic errors in order that the two types of errors can be separately compensated.
It is an object of this invention to provide a multiple beam printhead such that proper chromatic operation is achieved while utilizing lasing sources of significantly different wavelength.
It is still another object of this invention to provide an electronic correction scheme utilizing relatively low clock frequencies, but providing correction resolution to within one or two nanoseconds.
It is still another object of this invention to provide a chromatic correction scheme which adjusts the clock across the entire scan.
It is still another object of this invention to provide an electronic correction scheme which is adaptive to dynamic variations in wavelength which may occur due to temperature or age.